1. Field of the Invention
This invention relates to a switchable permanent magnet holding device for holding or being held by a magnetic substance.
2. Description of the Prior Art
One of the conventional permanent magnet holding devices of the type, as generally shown at 10 in FIG. 1, includes a magnetic circuit block 16 consisting of a pair of magnetic pole members 12 and a non-magnetic plate 14 disposed between the pair of magnetic pole members, and a columnar permanent magnet 18 rotatably disposed within a bore running through the magnetic block.
The permanent magnet 18 is magnetized in the diametrical direction thereof. When the permanent magnet 18 assumes a rotating position in which the direction of magnetization of the magnet runs in parallel to the non-magnetic plate 14, as shown in FIG. 1, then respective magnetic flux .phi. forms a close loop within the magnetic circuit block 16, without running across a magnetism acting surface 20 of respective magnetic pole member 12. Accordingly, the magnetism acting surfaces 20 remain non-exciting, when the permanent magnet 18 assumes that position, and hence the permanent magnet holding device 10 remains non-exciting.
When the permanent magnet 18 is turned, for example, counterclockwise substantially through 90.degree. from the non-exciting rotating position, the magnetism acting surfaces 20 become excited.
So far as the permanent magnet 18 assumes the non-exciting rotating position or the excited rotating position making a right angle with respect to the non-exciting rotating position, since the magnetic circuit block 16 is held in magnetical equilibrium, the permanent magnet 18 by no means receives a gyromagnetic force.
When the permanent magnet 18 assumes an intermediate position between the exciting rotating position and the non-exciting rotating position, as shown in FIG. 2, the magnetic equilibrium of the magnetic circuit block 16 is broken, and the permanent magnet 18 receives a gyromagnetic force. In the graph of FIG. 2, the axis of abscissa represents an angle of rotation (.theta.) of the permanent magnet 18 from the non-exciting rotating position shown in FIG. 1, and the axis of the ordinate represents a gyromagnetic force T which the permanent magnet 18 receives. The graph represents a variation in a gyromagnetic force which occurs when a magnetic substance is abutted on the magnetism acting surfaces of the block 16. From this graph, it is seen that when the permanent magnet 18 is located at an exciting rotating position which makes a right angle with respect to the first non-exciting position, the permanent magnet 18 by no means receives a gyromagnetic force toward the first non-exciting rotating position located at a zero degree or the second non-exciting rotating position located at 180.degree. which is opposite thereto. By displacement to some degree of the permanent magnet 18 from the exciting rotating position located at 90.degree. in rotational angle, the permanent magnet 18 receives a strong gyromagnetic force toward the non-exciting rotating position located at a zero degree or that located at 180.degree. to which the permanent magnet 18 has been displaced.
Because of such a structure, a problem has been encountered with the conventional permanent magnet holding device, in which when the permanent magnet holding device 10 receives shock, the permanent magnet 18 located at the excited rotating position is displaced therefrom to some degree and turned toward one of the non-exciting rotating positions due to the gyromagnetic force, as a result of which the permanent magnet holding device becomes unexpectedly non-exciting thus leading to an accident.
Such trend has been marked, particularly with a permanent magnet holding device of the type, in which, with a view to reducing a working force for rotating the permanent magnet 18, the sliding surfaces of the permanent magnet 18 relative to the magnetic circuit block 16 make smooth so as to decrease the mechanical frictional force therebetween. In such a device, the permanent magnet 18 is easily displaced from the exciting rotating position even by a feeble external force such as oscillation, as a result of which the holding device becomes suddenly non-exciting.